The wireless access scheme for the next-generation mobile communication system to the 3rd generation is currently discussed by 3rd Generation Partnership Project (3GPP), for example. The next-generation mobile communication system may be referred to as Long Term Evolution (LTE). It may also refer to as Evolved UTRA and UTRAN.
A mobile communication system in which LTE is used as its wireless access scheme does not use dedicated channel resource allocation, which has been used, for example, for Wideband Code Division Multiple Access (W-CDMA). For the mobile communication system in which LTE is used as its wireless access scheme, specification is discussed on the basis of resource allocation by means of shared channel. The “shared channel” is a radio channel shared by multiple users. This allows limited radio resources to be utilized efficiently.
[Non patent document 1] 3GPP TS 36.211 V.8.2.0 March 2008
For the wireless communication system in which LTE is used as its wireless access scheme, a base station (BS) transmits downlink signals including a reference signal (RS). The reference signal may be mapped as depicted, for example, in FIG. 1. FIG. 1 illustrates an exemplary mapping of reference signals. In FIG. 1, the abscissa axis indicates frequency and the ordinate axis indicates time. For example, one radio frame includes ten subframes. One subframe includes two slots. One slot may include six or seven OFDM symbols. FIG. 1 illustrates that each subframe has reference signals for unicast mapped thereto with six-subcarrier interval (one reference signal in every six subcarriers). FIG. 1 also illustrates that each slot has reference signals for MBMS mapped thereto with two-subcarrier interval (one reference signal in every two subcarriers). Besides reference signals for unicast and reference signals for MBMS, resource elements (RE) mapped to the frame may include data (data symbols). One resource element may be a radio resource with one OFDM symbol and one subcarrier.
A mobile station may perform channel estimation based on the reference signals included in the downlink signals transmitted from the base station (BS).
The reference signals, however, may be mapped without taking radio environment into account. Consequently, if demodulation is performed based on such reference signals, modulation accuracy may be degraded. FIG. 2 illustrates a relation between subcarriers and their Error Vector Magnitude (EVM) in a mobile station, the subcarriers being demodulated from received signal via an antenna. The subcarriers may be at a base band frequency. FIG. 2 depicts that the EVM degrades more at both edges of the band. As a result, reception performance also degrades.
For example, the radio unit of the reception unit in the mobile station may filter out signals from other users located near the mobile station with a baseband filter. The baseband filter may include a Low Pass Filter (LPF). If the filter slope (attenuation factor) of the baseband filter is to be steep (increased), the radio unit may be larger. That means that the filter slope may need to be reduced for smaller circuit of the radio unit. In the case of reduced filter slope, the baseband filter may cut a portion of band (hereinafter, may be referred to as a signal band) in which the signal is transmitted.
For example, a Wideband Code Division Multiple Access (W-CDMA) system uses a 3.84 MHz wide signal band. In this case, a wide gap (hereinafter, referred to as adjacent channel gap) may be reserved between adjacent signal bands. Thus, for a W-CDMA system, the signal band is seldom cut even in the case of reduced filter slope of the baseband filter. Consequently, filter characteristics only affects the modulation accuracy (EVM) to a small extent.
On the other hand, a LTE system uses 4.5 MHz wide signal band. It uses narrower adjacent channel gap than the W-CDMA system does. Thus, for the LTE system, the signal band may be cut in the case of reduced filter slope of the baseband filter. Consequently, filter characteristics affects the modulation accuracy (EVM) to a great extent. For example, as illustrated in FIG. 3, a baseband filter cuts the edge of signal band for a user (1). In FIG. 3, a signal portion that is cut by the baseband filter is indicated by shaded area. As a result, assuming the relation between subcarriers and their EVMs as illustrated in FIG. 2, the modulation accuracy of a signal may appear to be degraded due to the effect of the baseband filter illustrated in FIG. 3 regardless of the actual reception condition of the signal which is good. A reception unit transmits the reception quality of received signal from a transmission unit, to the transmission unit.
For example, a reception unit transmits the reception quality as a Channel Quality Indicator (CQI). For example, a mobile station with a receiver unit generates CQI based on downlink signal transmitted from the transceiver unit of a base station, and transmits the CQI to the base station. The base station determines communication condition with the mobile station on the basis of the transmitted CQI from the mobile station. If the base station performs scheduling based on the degraded portion of reception quality in the signal band illustrated in FIG. 3, the base station may allocate less radio resources to the mobile station than otherwise allocated based on true reception quality. The true reception quality is a reception quality without the effect of the baseband filter. For example, if the based station receives a CQI corresponding to a band portion including the center of signal band and another CQI corresponding to another band portion including the edge of the signal band, the base station may communicate with the mobile station based on the CQI corresponding to the band portion including the edge of the signal band. This is because the CQI corresponding to the band portion including the edge of the signal band appears to indicate degraded reception characteristics due to the effect of baseband filter characteristics. Consequently, the mobile station may not communicate with the base station at an otherwise-available transmission rate (throughput).